Membranes with different hydration levels: The interface between bound and unbound hydration water

The interaction of water with membranes is fundamental in many biological processes. Recently we found that, upon increasing hydration, water molecules first fill completely the interior of the membrane, next accumulate in layers in the exterior region. Here, we show by all-atom simulations that the translational and rotational dynamics of water molecules is strongly determined by their local distance to the membrane so that we can identify the existence of an interface between the first hydration shell, partially made of hydration water bound to the membrane, and the next shells entirely made of unbound hydration water. Bound hydration water has a possible structural role and an extremely slow dynamics, while unbound hydration water, with no water-lipids hydrogen bonds, has a dynamics ten time faster than bound water but still one order of magnitude slower than bulk water. Our results could be relevant to understand the slowdown of biological activity upon dehydration. (C) 2018 Elsevier B.V. All rights reserved.